1. Field of the Invention
The present invention relates to a gold alloy thin wire, utilized for connecting electrodes on a semiconductor device to external leads, excellent in the reliability of bonded portions.
2. Description of the Related Art
A gold alloy thin wire is principally used as a bonding wire for bonding electrodes on a semiconductor device to external leads.
Thermosonic wire bonding is generally used as the technology for bonding a gold alloy thin wire. The thermosonic bonding system is a method which includes heating and melting the tip of a gold thin wire with an arc to form a ball due to surface tension, compression bonding the ball portion to an electrode of a semiconductor device heated to a temperature from 150 to 300.degree. C., and connecting the external lead side of the wire to the external lead by ultrasonic compression bonding.
In order to use the semiconductor device as a transistor, an IC, and the like, the Si chip, the bonding wires, and the portion of the lead frame to which Si chip has been attached are sealed with a thermosetting epoxy resin to protect these parts.
The properties a gold thin wire should have are diversified due to the tendency of semiconductors toward becoming highly integrated and thin. For example, the gold alloy thin wire is required to be made long and thin or to be capable of forming a high loop so that the wire can cope with high density and narrow pitch wiring. Moreover, the wire is required to be capable of forming a low loop so that the semiconductor devices can be made thin. In order to cope with the problems of making the wire long and thin and adjusting the loop height, gold thin wires to which several alloying elements are added have been developed and are disclosed, for example, in Kokai.(Japanese Unexmined Patent Publications) No. 61-296731 and No. 61-172343.
The environmental conditions under which semiconductors are used have become very severe recently. For example, semiconductor devices employed within the engine compartments of automobiles are sometimes used in a high temperature or high humidity environment. Moreover, since semiconductor devices are mounted at high densities, the heat generated thereby during use must not be neglected. Furthermore, when a gold alloy thin wire is used, a decrease in the long term reliability of bonded portions with aluminum electrodes, etc. in a high temperature environment has become a problem.
For semiconductor devices used under environmental conditions requiring the devices to have heat resistance, an aluminum alloy thin wire has heretofore been used as a bonding wire, and semiconductor devices packaged in ceramic have been utilized. The aluminum alloy thin wire has the advantage that high reliability in the bonded portions with electrodes on the semiconductor devices can be obtained due to the bonding of metals of the same type. However, ceramics packaging is costly compared with resin sealing, and the aluminum alloy thin wire cannot easily form a normal ball in the air. Accordingly, wedge bonding is commonly practiced, and the productivity of semiconductors is lowered compared with the use of a gold alloy thin wire.
The use of an aluminum alloy thin wire is restricted to specific semiconductors for reasons of cost and productivity. A bonding system with a gold alloy thin wire excellent in high speed bondability, productivity, processability, etc. will continue to be a major process still in the future. In bonding a gold thin wire and the aluminum electrodes together, a gold alloy thin wire having high bonding reliability in a high temperature environment is desired in related industrial fields.
When a conventional gold thin wire is used, a decrease in the long term reliability of bonded portions with aluminum electrodes on semiconductors has been a problem. That is, it has been pointed out, as a problem, that formation of intermetallic compounds and generation of voids resulting from mutual diffusion of the electrode material aluminum and gold cause the peeling of the bonded portions and poor electric conduction therein.
As a result of investigating the reliability of the bonded portions between gold alloy thin wire and aluminum electrodes, the present inventors have confirmed that corrosion of the intermetallic compounds in the resin sealed bonded potions greatly influences the reliability. The reaction of the intermetallic compounds of gold and aluminum, which have grown near the bonded interface, with a halogen component contained in the resin sealing increases the electric resistance of the bonded portions and produces poor electric conduction at the time of significant corrosion.
Since the halogen component is essential as a flame-retardant of the resin sealing, the growth of the corrosive gold/aluminum compound layer must be inhibited for the purpose of decreasing the corrosion of the intermetallic compound layer.
In order to control the diffusion behavior in the bonded portions for the purpose of improving the reliability, addition of alloying elements to the gold thin wire may sometimes be effective. The present inventors also have found elements effective in inhibiting the corrosion, and disclosed the control of the compound phase by adding a Mn element in Kokai (Japanese Unexamined Patent Publication) No. 6-824322. As the amount of addition of Mn increases, oxidation of the thin wire during the formation of a ball in the air causes a problem As a result, the upper limit of the addition amount of the Mn element is restricted. Moreover, it is difficult to completely inhibit a decrease in the bonding strength in the initial stage of corrosion by only adding the Mn element in a small amount. The difficulty of inhibiting corrosion becomes a problem.